Optical molasses

Optical molasses is a laser cooling technique that can cool neutral atoms to as low as a few microkelvin, depending on the atomic species. An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the region where the atoms are present. The main difference between optical molasses and an MOT is the absence of magnetic field in the former. Therefore, unlike a MOT, an optical molasses provides only cooling and no trapping.

History
When laser cooling was proposed in 1975, a theoretical limit on the lowest possible temperature was predicted. Known as the Doppler limit, $$ T_d= \hbar \Gamma / {2 k_b} $$, this was given by the lowest possible temperature attainable considering the cooling of two-level atoms by Doppler cooling and the heating of atoms due to momentum diffusion from the scattering of laser photons. Here, $$ \Gamma $$, is the natural line-width of the atomic transition, $$ \hbar   $$, is the reduced Planck constant and, $$ k_b  $$, is the Boltzmann constant.

Experiments at the National Institute of Standards and Technology, Gaithersburg, found the temperature of cooled atoms to be well below the theoretical limit. In 1988, Lett et al. directed Sodium atoms through an optical molasses and found the temperatures to be as low as ~40μk; 6 times lower than the expected 240μk doppler cooling limit. Other unexpected properties found in other experiments included significant unexpected insensitivity to laser alignment of the counter-propagating beams.

Theory
The best explanation of the phenomenon of optical molasses is based on the principle of polarization gradient cooling. Counterpropagating beams of circularly polarized light cause a standing wave, where the light polarization is linear but the direction rotates along the direction of the beams at a very fast rate. Atoms moving in the spatially varying linear polarisation have a higher probability density of being in a state that is more susceptible to absorption of light from the beam coming head-on, rather than the beam from behind. This results in a velocity dependent damping force that is able to reduce the velocity of a cloud of atoms to near the recoil limit. The recoil limit, $T_r$, is set by the energy of the photon emitted in the decay from the J’ to J state, where the J state is the ground state angular momentum and the J’ state is the excited state angular momentum. This temperature is given by $$k_bT_r=\frac{h^2}{M\lambda^2}$$ though practically the limit is a few times this value because of the extreme sensitivity to external magnetic fields in this cooling scheme. Atoms typically reach temperatures on the order of $\mu K$, as compared to the doppler limit $T_D\simeq240\mu K$.